A rechargeable secondary battery unit is used for a mobile notebook personal computer (PC). At present, a fuel cell unit cannot provide sufficient current for a notebook PC when it is used alone. Accordingly, it has been proposed to use a secondary battery unit in combination with a fuel cell unit for a notebook PC.
Japanese Patent Application Publication JP 2004-319367-A laid open to the public on Nov. 11, 2004 discloses a battery pack. For providing an appropriate voltage to a load, the battery pack has a battery and a DC-to-DC converter built therein. When the battery pack is connected to the load through a connector, and when an output voltage indicative value output means operates to set a value indicative of an output voltage corresponding to a supply voltage for the load, the DC-to-DC converter converts an output voltage of the battery in accordance with the output voltage indicative value. A voltage resulting from the conversion is applied to the load through an output voltage line.